1. Field of the Invention
This invention relates generally to the hydrogenation of unsaturated organic compounds, and especially to the hydrogenation of .alpha.-acids and the hydrogenation/hydrogenolysis of .beta.-acids. More specifically, it relates to the purification of .alpha.-acids and .beta.-acids by removing catalyst poisons so that the hydrogenation reaction is more efficient and economical.
2. Background of the Art
Catalyst poisoning is a common and vexing problem with noble-metal catalysts which are used in a variety of industrial settings. Catalyst poisoning can cause difficulties such as higher catalyst loadings or reaction failures which result in higher operating costs. The problem of catalyst poisoning extends to processes for hydrogenating .alpha.-acids and .beta.-acids.
We have not identified the catalyst poisoning compounds present in hop extracts, but we believe some sulfur compounds (especially disulfides) are in the hop oil fraction and are responsible for deactivation of the catalyst. One approach for minimizing the presence of catalyst deactivation compounds is the use of an organic solvent-free process for the separation and purification of .alpha.-acids and .beta.-acids from the hop oil fraction in hop extracts. See U.S. Pat. Nos. 4,590,296 and 4,644,084. The hydrogenation/hydrogenolysis of unpurified .beta.-acids from such a process typically requires a 10% loading for a 5% Pd/C catalyst based on the feed material. However, when hexane (or other organic solvent)-recrystallized .beta.-acids are used as feed material the same catalyst can be used at a 1-2% loading. Thus, such improvement in the purity of organic solvent-free .beta.-acids which further reduces catalyst poisons is highly desirable. Because catalysts are a very expensive commodity, a reduction in their use results in significant savings.
Efforts have been made to remove the catalyst poisons by further purification of .beta.-acids prior to the hydrogenation/hydrogenolysis reactions. For example, U.S. Pat. No. 4,918,240 discloses that .beta.-acids, separated from hop extracts, can be purified prior to hydrogenolysis by mixing with an aqueous solution of polyvalent metal ions, separating the aqueous .beta.-acids from insoluble materials containing the catalyst poisons, and recovering the thus purified .beta.-acids which are catalyst-poison free and in a form suitable for hydrogenolysis to tetrahydro-4-desoxy-.alpha.-acids. However, this method is tedious (including pH control and multiple treatments with divalent salts), time consuming, and uses organic solvents.
Thus, it would be advantageous to have a relatively simple process of removing catalyst poisons from the .alpha.-acid and .beta.-acid streams so that the hydrogenation reaction is more efficient and economical.